Crucible

ABSTRACT

A resistance heating crucible for use in holding a sample to be fused by a furnace includes a cylindrical side wall communicating with a bottom floor having an exterior button-like projection extending centrally therefrom to define an annular contact surface surrounding the button-like projection for seating with the crucible-activating electrode associated with the furnace. The circular bottom is chamfered along the junction of the bottom with the cylindrical side wall to reduce and limit the annular contact surface. the interior floor of the crucible is rounded to reduce the cross section through the bottom and in cooperation with the chamfered edge, increases the heating effect of current passing through the bottom of the crucible.

. United States Patent [191 I Sitek et a1.

[ CRUCIBLE [75] lnventors: George J. Sitek, Stevensville;

Charles W. Berk, St. Joseph, both of Mich.

[73] Assignee: Leco Corporation, St. Joseph, Mich.

[22] Filed: June 28, 1974 [21] Appl. No.: 484,303

[ 1 Aug. 12, 1975 Primary Examiner-R. N. Envall, Jr. Attorney, Agent, orFirm-Price, l-leneveld, l-luizenga & Cooper [5 7 1 ABSTRACT A resistanceheating crucible for use in holding a sample to be fused by a furnaceincludes a cylindrical side wall communicating with a bottom floorhaving an exterior button-like projection extending centrally therefromto define an annular contact surface surrounding the button-likeprojection for seating with the crucible-activating electrode associatedwith the furnace. The circular bottom is chamfered along the junction ofthe bottom with the cylindrical side wall to reduce and limit theannular contact surface. the interior floor of the crucible is roundedto reduce the cross section through the bottom and in cooperation withthe chamfered edge, increases the heating effect of current passingthrough the bottom of the crucible.

10 Claims, 4 Drawing Figures CRUCIBLE BACKGROUND OF THE INVENTION Thepresent invention relates to crucibles for fusion of a specimen andparticularly to crucibles made of a resistive material.

In resistance furnaces for fusing specimens to obtain gas samplestherefrom of the type manufactured by Leco Corporation of St. Joseph,Michigan, as an integral part of an analyzer, Model No. TC-30, graphitespecimen-holding crucibles are employed for containing the sample. Thecrucible is positioned between actuating electrodes for heating thesample to a fusion temperature. Several crucible designs have beenpreviously employed in such furnaces including a stud crucible of thetype described in U.S. Pat. No. 3,636,229 assigned to the presentassignee.

Although prior crucibles have worked well for certain specimens such asoxygen and adequately for other specimens, it has been discovered thatwith some specimen gases such as nitrogen which require significantenergy to cause the required reduction reaction with the carboncrucible, existing crucibles have not performed as well as desired.

SUMMARY OF THE INVENTION It was discovered that the crucible bottom canbe configurated to reduce the electrical current crosssectional areathereby increasing the direct heating of the bottom instead of throughconduction of a heated stud as disclosed in the above identified patent.The increased efficiency in directly heating the crucible floor forfusing the specimen permitted the improved crucible design disclosedherein to be employed with the same furnace having the same maximumcurrents as previously used but with considerably more success withspecimen gases such as nitrogen. Thus, by virtue of the improvedcrucible design, for the same amount of applied power, significantlygreater efficiency of fusion of the sample is provided.

The crucible design of the present invention also increases thedispersion of the molten specimen during fusion to further reduce thespecimen as it reacts with the carbon crucible. Also, reduced furnacepower can be employed with many specimens. Finally, in some cases,fluxes previously required to achieve complete fusion can be eliminated.

Crucibles embodying the present invention include a cylindrical sidewall communicating with an integral bottom. The junction between theside wall and the outside surface of the crucible bottom is chamfered toreduce the contact area between the crucible bottom and the furnaceelectrode supporting the crucible. The floor of the crucible bottom isrounded which, in cooperation with the chamfered side walls, reduces thecross-sectional area of the current path through the bottom of thecrucible thereby increasing the heating of the bottom by current flowingtherethrough. In one embodiment, the bottom includes an exterior button-BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentarycross-sectional view of a resistance heating furnace and the crucibleembodying the present invention shown positioned in relation to thefurnace electrodes;

FIG. 2 is an enlarged cross-sectional view of the crucible shown in FIG.1;

FIG. 3 is a top plan view of the crucible shown in FIG. 2; and

FIG. 4 is a bottom plan view of the crucible shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thefigures, crucible 10 embodying the present invention is made from a purecarbon having a density of 1.8 grams per cubic centimeter and a specificresistance for a cross-sectional rod of 0.56 inch in diameter of 0.00042ohms per inch. The material is commercially available and identified asArco Speer carbon, Grade No. 7lOGL. The crucible includes a cylindricalside wall 12 having an open top 14 for introducing the specimen 11(FIG. 1) and an integrally formed bottom 16 defining at the interiorofthe crucible, floor 18.

Floor 18 is rounded while the junction 20 of the exterior of the sidewall 12 with the bottom of the crucible is chamfered around itsperiphery to reduce the contact area of the bottom surface 22 of thecrucible. A buttonlike cylindrical projection 24 extends downwardly fromthe axis of the crucible and is employed for centering the crucible onthe lower electrode as described below.

The electrical contact area for the crucible bottom with the lowerelectrode is the horizontally extending surface 22 which, in thepreferred embodiment, is an annular ring as best seen in FIG. 4. The toprim 13 (best seen in FIG. 3) of the crucible also defines an annularcontact surface for engagement with the upper electrode. The concavelyrounded bottom 18 and the chamfered lower edge 20 of the cruciblereduces the electrical current cross section path through the bottom ofthe crucible significantly and as is well-known, reducing thecross-sectional area of a conductive material increases its resistance.Thus, the PR heating effect within the bottom of the crubile itself isgreatly increased to directly heat the bottom and increase theefficiency of operation of the crucible over prior crucible designs. Theconfiguration of the furnace electrodes and the tips therefor isdisclosed in detail in a copending U.S. patent application entitledELECTRODE CONSTRUCTION FOR RESISTANCE HEATING FURNACE, Ser. No. 484,453filed concurrently herewith, assigned to the present assignee andincorporated herein by reference. A brief description, however, ispresented here to more fully describe the operation of the improvedcrucible.

The crucible is supported on the pedestal 32 of a lower copper electrode30 movable toward and away from upper electrode 40 to alternately clampand release the crucible between the electrodes respectively. Pedestal32 includes a tip 34 having a significantly annular mating surface 35configurated to seat against surface 22 of the crucible. Tip 34 alsoincludes a central aperture 36 for receiving the centering projection 24of the crucible and a plurality of cleaning notches 37 extending fromaperture 36 radially outwardly through the otherwise continuous annularsurface 35. Tip 34 is made of tungsten or a pressed mixture of 75%tungsten and 25% copper.

The lower electrode includes an annular recess 38 formed downwardlytherein for receiving the downwardly projecting end 42 of the upperelectrode. Sealing means 43 comprising O-rings extend around the lowerperipheral edge of portion 42 for sealably fitting within recess 38 whenthe lower electrode 30 is moved upwardly in a direction indicated byarrow A by the control shaft 39 attached thereto.

The upper electrode includes a cylindrical cruciblereceiving chamber 44extending upwardly therein and communicating with a cylindricalpassageway 45 for introducing a sample into the crucible as well as acarrier gas such as helium. Extending around the junction of passageways44 and 45 is a washer-shaped insert 46 of the same material as tip 34and which seats against rim 13 of the crucible. Tip 46 includes radiallyextending slots 46 to permit specimen gas and carrier gas to bedischarged from the crucible during fusion. A discharge port 47 extendsfrom the bottom of electrode 40 upwardly and communicates with adischarge tube 48 for transporting the specimen gas to the analyzerassociated with the furnace. It is noted that the open end of the bottomof port 47 does not contact the floor of recess 38 such that gases arepermitted ,to discharge through coupling 48 during analysis. A powersupply 50 provides alternating current for actuating the electrodes topass current through the crucible when the electrodes are in the closedposition thereby clamping the crucible between inserts 34 and 46.

In the preferred embodiment, the crucible is integrally formed bymachining a solid rod of the carbon material noted above and thecylindrical side wall has an outer diameter of 0.56 inch and an innerdiameter of 0.48 inch. The rounded bottom 18 has a 0.6 inch radius ofcurvature and the chamfer angle is 30. The button-like projection 24 hasa diameter of 0.18 inch while the outer diameter of annular surface 22is 0.31 inch. The height of the crucible from the floor 22 to rim 13 is0.625 inch while the button-like projection 24 extends downwardlyapproximately 0.055 inch. The thickness of the crucible bottom 16 at BBis 0.2 inch. This dimension is significantly reduced over existingcrucibles by the smoothly curved concave floor 18 and chamfer 20 andaccounts for the reduced electrical current cross-sectional area. Thesedimensions are approximate and subject to normal tolerances of about Thearea of annular contact surface 22 is about 0.005 square inches whilethe internal cross section of the bottom in area B-B is approximatelythe mean diameter of 0.4 inch multiplied by the width of 0.2 inch and11' or about 0.25 square inches.

It is seen in FIG. 1 that the bottom of the projection 24 does notcontact the tip 34 of the electrodes 30 but serves only a centeringfunction with the sole contact area between the electrode and thecrucible being the annular surface 22. Should the crucible tip 34 wearafter considerable use and the contact and the annular surface 35 of thetip become enlarged, the contact area with each crucible (one is usedfor each analysis) remains the same by virtue of the chamfer whichprevents the contact area from increasing. It is noted that in someembodiments, the centering stud 24 may not be necessary and the cruciblecould be flat-bottomed. In such embodiments, however, the chamfered edge20 is required to. reduce the bottom electrical current cross sectionarea. These and other modifications to the preferred embodiment will,however, come within the spirit and scope of the present invention asdefined by the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

1. A resistance heating crucible made of a conductive material forfusion of a specimen and comprising a cylindrical side wall having anopen end defining the top of said crucible and an enclosed end definingthe bottom of said crucible, wherein the interior of said bottom isconcavely rounded and the junction of the exterior of said side wallwith the bottom is chamfered to reduce the cross-sectional area of theelectric current path through said bottom of said crucible.

2. The crucible as defined in claim 1 and further including abutton-like projection extending downwardly from the center of saidcrucible bottom.

3. The crucible as defined in claim 2 wherein said junction is chamferedat an angle of 30.

4. The crucible as defined in claim 1 wherein the exterior of saidbottom is configurated to define a horizontally extending annularcontact area.

5. The crucible as defined in claim 1 wherein said interior of saidbottom is rounded at about 0.6 inch radius of curvature and saidcross-sectional area is a maximum of about 0.25 square inches.

6. The crucible as defined in claim 5 and further including acylindrical projection extending downwardly from the center of saidbottom about 0.05 inch and having a diameter of about 0.19 inch.

7. In combination with the crucible as defined in claim 2, an electrodefor supporting said crucible including a substantially annular contactsurface surrounding said projection and a recess into which saidprojection extends, said recess having a depth greater than the lengthof said projection.

8. A carbon crucible for use in holding and heating a solid specimenbetween conductive electrodes of a furnace, said crucible including anopen top, a cylindrical side Wall and an enclosed bottom, wherein theimprovement comprises: said bottom including a concavely roundedinterior defining the floor of said crucible and the junction of saidbottom and said cylindrical side wall significantly chamfered around theperiphery of said crucible to reduce the surface area of said bottom.

9. The crucible as defined in claim 8 and further including acylindrical button-like projection extending downwardly from the centerof said bottom.

10. For use in a fusion furnace, a carbon crucible comprising: acylindrical side wall open at one end and enclosed at an opposite end todefine a crucible bottom wherein the interior of said bottom isgenerally concavely curved and the exterior of said bottom is circularand chamfered around the edge adjoining said side wall to reduce thecross-sectional area of the current flow path through said cruciblebottom.

1. A resistance heating crucible made of a conductive material forfusion of a specimen and comprising a cylindrical side wall having anopen end defining the top of said crucible and an enclosed end definingthe bottom of said crucible, wherein the interior of said bottom isconcavely rounded aNd the junction of the exterior of said side wallwith the bottom is chamfered to reduce the cross-sectional area of theelectric current path through said bottom of said crucible.
 2. Thecrucible as defined in claim 1 and further including a button-likeprojection extending downwardly from the center of said crucible bottom.3. The crucible as defined in claim 2 wherein said junction is chamferedat an angle of 30*.
 4. The crucible as defined in claim 1 wherein theexterior of said bottom is configurated to define a horizontallyextending annular contact area.
 5. The crucible as defined in claim 1wherein said interior of said bottom is rounded at about 0.6 inch radiusof curvature and said cross-sectional area is a maximum of about 0.25square inches.
 6. The crucible as defined in claim 5 and furtherincluding a cylindrical projection extending downwardly from the centerof said bottom about 0.05 inch and having a diameter of about 0.19 inch.7. In combination with the crucible as defined in claim 2, an electrodefor supporting said crucible including a substantially annular contactsurface surrounding said projection and a recess into which saidprojection extends, said recess having a depth greater than the lengthof said projection.
 8. A carbon crucible for use in holding and heatinga solid specimen between conductive electrodes of a furnace, saidcrucible including an open top, a cylindrical side wall and an enclosedbottom, wherein the improvement comprises: said bottom including aconcavely rounded interior defining the floor of said crucible and thejunction of said bottom and said cylindrical side wall significantlychamfered around the periphery of said crucible to reduce the surfacearea of said bottom.
 9. The crucible as defined in claim 8 and furtherincluding a cylindrical button-like projection extending downwardly fromthe center of said bottom.
 10. For use in a fusion furnace, a carboncrucible comprising: a cylindrical side wall open at one end andenclosed at an opposite end to define a crucible bottom wherein theinterior of said bottom is generally concavely curved and the exteriorof said bottom is circular and chamfered around the edge adjoining saidside wall to reduce the cross-sectional area of the current flow paththrough said crucible bottom.